American Siren with 555

Discussion in 'Homework Help' started by PsySc0rpi0n, Oct 26, 2015.

1. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Hello once more...

The last problem I have to solve for this pre-lab assignment job is to build an "american siren" using the 555.

I did some research and found a few possibilities for this. I tried to pick one that looked more simple to me!
I built it in LTSpice and I think it's working.
But I need to know if anyone can come up with something even more basic and simple so that I can understand it easily!

I'm going to calculate the tON and tOFF for this circuit as the oscillation frequency!!

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2. WBahn Moderator

Mar 31, 2012
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What is an "American siren"?

You mentioned a "two-tone" siren in an earlier thread. I associate a siren that alternates between two tones with European sirens. Most of the sirens I hear here have a tone that slides up and down in frequency fairly smoothly.

3. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Well, I'm also not sure about what would be an american siren, but I presume it will be a 2 tone sound...

Anyway, I have calculated the tON and tOFF for both 555 outputs and also the oscillation frequency, but I found different values compared to those of who claimed to built the circuit.

From that site, I quote:

I have calculated tON_1 = ln (2) * 68kΩ * 10μ = 471 ms
As it's an astable mode with one diode, it is possible to get 50% Duty Cycle, as it's mentioned in the quote. So tOFF_1 = 0.471 ms, roughly.

The frequency is also roughly 1Hz but the quote claims to be about 20Hz. I can't understand if it was a mistake from the one who built the circuit of if it's some mistake of mine!

For the right 555

tON_2 = ln (2) * (8.2kΩ + 8.2kΩ) * 100n = 1.137 ms
tOFF_2 = ln (2) * 8.2kΩ * 100n = 0.568 ms

f = 587Hz

For the right 555 it looks like that calcs match but I can't understand why I get 1Hz for the left 555 and that site says it's about 20Hz.
Also I would like some help to understand what is the role of C4.

4. JoeJester AAC Fanatic!

Apr 26, 2005
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1,218
Here's a siren featured in NYC at NYPD

5. WBahn Moderator

Mar 31, 2012
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20 Hz seems too fast. I would imagine something along the lines of a few hertz is more reasonable.

Apr 30, 2011
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As usual, Whelen Engineering has a demonstrator online:

http://www.whelen.com/auto/sirentones.php

The 'Simulated Mechanical' is the old American classic siren sound. The 'Wail' is the early electronic siren sound. Others such as the 'Yelp', 'Piercer' and 'Growler' are common now. This demonstrator doesn't include a classic European sound but the 'HiLow' is somewhat similar.

7. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Ok guys, we don't need to be very thorough about the 2 tone siren. I think the purpose would be to generate a non-single frequency output waveform using the 555. Whether is an american, european, police or fire department siren, won't make much of a difference (my guess).

I just wanted to know if the 20Hz is a mistake of whoever built the circuit or if it was a mistake of mine!
Also wanted to know about the role of C4 as I have no idea!

8. ericgibbs AAC Fanatic!

Jan 29, 2010
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hi Psy,
Have you noticed the Siren out signal swings positive and negative about zero, how do you think is swing is achieved.?

Also examine the signal at the 555 output pin.

E

9. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
1,191
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Ok, I think I got it. I'm going to try to explain it, without calcs, for now!

The whole idea is that the left 555 modulates the CV pin of the right 555. So, the left 555 will be set up to have 2 different levels of voltage at the output pin. This will set 2 different threshold voltages to CV pin of the right 555. By correctly setting up the 2 external resistors and timing caps of both 555 we will make the left 555 frequency, a low frequency and the right 555, a high frequency so that the overall output (right 555 output) can work for some time (set by left 555) at some frequency (set by right 555).

So, giving an example:
Let's assume:

Left 555
Freq_left = 1 Hz
Threshold_1 = x V
Threshold_2 = y V

for these conditions, right 555:

Freq_right_1 - set by Threshold_1 or _2 for the duration of Freq_left
Freq_right_2 - set by Threshold_1 or _2 for the duration of Freq_left

This will result in a sound with 2 frequencies, each of duration of the left 555 frequency.

PS: EricGibbs, I'm not sure why the voltage crosses the x-axis... So I don't know the answer to your question!

PS2: Ok, a friend of mine gave me a tip about the C4 role! It stops any DC voltage to go across the speaker/resistor! So if I plot the voltage at the output pin, before the cap and the resistor/speaker, I think I see an offset (DC component) and there is no negative values for the voltage because of that DC component that is pushing the wave upwards. So, the cap is removing that DC component and therefore, the final waveform is pulled downwards by that DC value, hence, there is a swing over and below the x-axis!

Last edited: Oct 27, 2015
10. ericgibbs AAC Fanatic!

Jan 29, 2010
2,595
393
hi,
Some Sirens and Sounders require a drive signal voltage that swings positive and negative, its effectively an AC drive voltage.

The current flowing into the C4 cap [when the 555 Out pin is +V] and out of the C4 cap , [when the 555 Out is 0V] changes direction thru the siren resistance, so the voltage across the siren changes from positive to negative.
E

11. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Ok, so the explanation I gave has nothing to do with what is really happening, right?

12. JoeJester AAC Fanatic!

Apr 26, 2005
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Your explanation is correct. Eric was pointing out the nature of the control signal to the second 555.

Attached is a nice paper on sirens.

• 2011_39_2_Howard.pdf
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13. Dodgydave AAC Fanatic!

Jun 22, 2012
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If you want it to sweep up and down, use a pnp transistor across the C1 cap and feed pin 5 of the second 555 timer with the emitter like this...

You will need to alter Rx circled to set the upper frequency, R3 sets the lower frequency, i would remove D1, and make R2 a variable 68k, then you can alter the sweep speed.

Last edited: Oct 27, 2015
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14. PsySc0rpi0n Thread Starter Well-Known Member

Mar 4, 2014
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Ok guys...

Thanks a lot for the help! I think this is another problem solved!